In metal halide high-pressure lamps having discharge vessels of quartz glass, not all optically active fill substances can be vaporized at operating temperature, so that some proportion thereof remains as a condensate within the discharge vessel, without participating in the discharge formation. The quartz glass, which can withstand the temperature only slightly above 1000.degree. C., provides an upper limit for the operating temperature. Quartz glass, further, is attacked by the highly aggressive fill substances used in metal halide high-pressure discharge lamps and becomes brittle due to recrystallization. This negatively affects the life of the lamp.
It has been proposed to utilize aluminum oxide ceramic as the bulb or arc vessel for a metal halide high-pressure lamp. Aluminum oxide ceramic is well known from the technology of sodium high-pressure lamps. This material is capable of withstanding a temperature of up to about 1300.degree. C., and is not attacked by the aggressive substances of the fill. Aluminum oxide ceramic, however, introduces a difficulty in the manufacture of a gas-tight, pressure-tight melt for the electrodes. Tungsten, which is customarily used for the current supplies, has a higher thermal coefficient of expansion than aluminum oxide ceramic. At the high temperatures which occur, the current supply lead made of tungsten would destroy the melt connection through the discharge vessel, if made of aluminum oxide ceramic.
It has been proposed to utilize current supply leads made of niobium rather than of tungsten. The thermal coefficient of expansion of nobium corresponds roughly to that of aluminum oxide ceramic, and the melt connection is not destroyed thereby. Niobium, however, has the disadvantage that it is attacked by the aggressive fill substances of the metal halides and thus the presence of niobium within the discharge vessel cannot be tolerated.
European published patent specification EP No. 0 074 188 describes a melt connection for a sodium high-pressure lamp in which the discharge vessel is made of aluminum oxide ceramic and a terminal connection for current supply of niobium is used. A current connecting lead of tungsten is welded to the niobium lead, the tungsten connection carrying the electrode. Melt connections with terminals made of Cermet are also proposed.
European published patent-specification EP NO. 0 074 720, assigned to the same assignee as the aforementioned EP 0 074 188, describes a further development in which the application of the current supply connection is extended to high-pressure discharge lamps with metal halide filling.
It has been found that the presence of even a portion of the niobium through-connection within the interior of the discharge vessel leads to deterioration thereof and premature failure of the lamp.